1. Field of the Invention
This invention relates to a gas turbine engine control system, particularly a control system for a stationary gas turbine engine.
2. Description of the Related Art
Gas turbine engines, particularly stationary gas turbine engines, are being developed as small units for connection with relatively small output generators to build on site (private) power generation systems. This type of gas turbine engine uses gas as fuel. The gas fuel and air are supplied to a burner to produce premixed combustion or diffusion combustion.
When these combustion modes are compared with regard to emission performance, premixed combustion is found to be superior from the viewpoint of low emissions. This is because the disadvantage of NOx emission concentration increasing with higher combustion temperature is offset by the advantage of being able to achieve combustion with a uniform temperature distribution in premixed combustion. However, the combustion temperature in premixed combustion is lower than that in diffusion combustion, meaning that the combustion is unstable and susceptible to flame failure, so that the temperature range in which premixed combustion is feasible is limited.
Although diffusion combustion is possible over the full range of combustion temperatures, the NOx emission concentration increases at scattered high-temperature regions. In the interest of reducing emissions, therefore, it is preferable to switch the combustion mode from diffusion combustion to premixed combustion when premixed combustion is possible.
With this in mind, U.S. Pat. No. 6,745,558, filed by the assignee, teaches a system that switches the combustion mode between premixed combustion and diffusion combustion in response to the combustion state, i.e., adiabatic flame temperature or air temperature at the inlet of the venturi mixer. The system set out in this reference is equipped with a first valve for regulating fuel used in premixed combustion and a second valve for regulating fuel used in diffusion combustion. The combustion mode switching is effected by controlling the openings of the two valves. Specifically, the opening of the second valve is controlled from the beginning to the end of combustion mode switching so that the part of all fuel supplied to the gas turbine engine which constitutes fuel for diffusion combustion is supplied at the minimum required for conducting diffusion combustion, and the opening of the first valve is controlled to supply the remainder as fuel for premixed combustion. The aim in conducting combustion in both modes while the switching of the combustion mode is in progress is to achieve stable switching. As will be noted in FIGS. 7 and 8 of this reference, the opening/closing of the valves is conducted simultaneously.
The fuel regulated by the first and second valves is supplied to the gas turbine engine through fuel pipes. The fuel supplied upon opening the first or second valve therefore reaches the burner after a delay proportional to the volume of the fuel pipe. Therefore, when the valves are opened/closed simultaneously at the beginning of combustion mode switching, the total amount of fuel supplied to the gas turbine engine (to its burner) momentarily falls below the desired amount to cause undesirable fluctuation of the engine speed.
In addition, in the case of the conventional arrangement wherein the part of the fuel supplied to the gas turbine engine for diffusion combustion is supplied at the minimum required for conducting diffusion combustion and the remaining part is supplied as fuel for premixed combustion, the change in the amount of fuel for diffusion combustion or for premixed combustion at the beginning or end of switching is relatively large. As a result, the total amount of fuel at the beginning and end of switching deviates from the desired value. This is liable to cause fluctuation of the engine speed.